1. Field of the Invention
The present invention relates to a bobbin around which a magnet wire is wound, to a motor in which the bobbin is incorporated, and further to a method of winding a magnet wire around the bobbin.
2. Description of the Related Art
FIG. 5 is a partly cross-sectioned axial view of a conventional stepping motor. The conventional stepping motor comprises a rotor assembly 10, and a stator assembly composed of two stator units 20 and 30.
The rotor assembly 10 is shaped substantially cylindrical, and composed of a permanent magnet with a plurality of magnetic poles 11 arranged on its outer circumference, and a rotary shaft 12 passing through the center of the magnet. The stator units 20 and 30 have respective center openings and are attached to each other coaxially, and the rotor assembly 10 is rotatably housed in the center openings.
The stator unit 20 includes yoke members 21 and 22 arranged to oppose each other, and a bobbin 24 having a magnet wire 23 wound therearound and sandwiched between the yoke members 21 and 22. The yoke member 21 has an opening corresponding to the center opening of the stator unit 20, and a plurality of pole teeth formed along its inner circumference and bent up toward the yoke member 22 so as to oppose the magnetic poles 11 of the rotor assembly 10. In the same way, the yoke member 22 has an opening corresponding to the center opening of the stator unit 20, and a plurality of pole teeth formed along its inner circumference and bet up toward the yoke member 21 so as to oppose the magnetic poles 11 of the rotor assembly 10.
The bobbin 24 includes a body section 24a, and flanges 24b and 24c provided at the both ends of the body section 24a, and a center hole with a constant diameter is formed through the body sections 24a, and the flanges 24b and 24c. The pole teeth of the yoke members 21 and 22 are inserted through the center hole of the bobbin 24 and intermesh with each other therein. An outer rim portion 22a of the yoke member 22 is bent, for example by drawing, toward the yoke member 21 for engagement therewith, thus constituting an outer circumferential wall of the stator unit 20.
The stator unit 30 includes yoke members 31 and 32 arranged to oppose each other, which are structured identical with the yoke members 21 and 22, respectively, and which sandwich a bobbin 24 having a magnet wire 23 wound therearound. An outer rim portion 32a of the yoke member 32 is bent, for example by drawing, toward the yoke member 31 for engagement therewith, thus constituting an outer circumferential wall of the stator unit 30.
A front plate FP is attached to a side of the yoke member 22 opposite to a side facing the yoke member 21, and one protruding end of the rotary shaft 12 of the rotor assembly 10 is rotatably supported by the front plate FP. A rear plate RP is attached to a side of the yoke member 32 opposite to a side facing the yoke member 31, and the other protruding end of the rotary shaft 12 of the rotor assembly 10 is rotatably supported by the rear plate RP.
As electronic devices are increasingly downsized, the stepping motor shown in FIG. 5 is requested to be downsized. One approach for answering the request is to reduce the diameter of the rotor assembly 10. This approach, however, is hitting a limit for the reason of angle resolution, and also torque characteristic.
Another approach is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H07-123686, in which the thickness of the outer rim portions 22a and 32a of the yoke members 22 and 32 are diminished, whereby the number of turns of the magnet wire 23 can be increased for increased torque when the motor size remains unchanged, or the motor can be downsized when the number of turns of the magnet wire 23 remains unchanged. This approach, however, raises a problem with mechanical strength, such as degraded reliability in protection against unforeseeable external forces. Also, when increased torque is required, the diameter of the flanges 24b, 24c of the bobbin 24 must be increased making it difficult to downsize the motor, thus preventing concurrent achievement of the downsizing and the increased torque.